The invention relates to a lamp, in particular metal halide discharge lamp having a protective sleeve surrounding an arc tube, wherein the sleeve and the arc tube are supported by a metal frame mounted to the stem, and a glass envelope fixed to the stem surrounds the sleeve.
Metal halide lamps typically incorporate a tubular shield surrounding the pressurized arc tube to absorb the impact of dispersing shards in the event the arc tube fractures. Both the shield and the arc tube are supported by a metal frame mounted to the stem, which frame is electrically isolated from the leads for the arc tube. This is especially important for high wattage lamps. Since current carrying members in proximity to the arc tube can cause sodium loss, it is preferable to electrically distance the current carriers from the lateral walls of the arc tube.
U.S. Pat. No. 6,153,968 discloses a metal halide lamp of the type described above, wherein an arc tube and the surrounding shield are supported by a metal frame mounted to the stem. The shield is a quartz sleeve whose ends are fitted with retaining rings which are welded to respective frame members on either side of the sleeve. The arc tube has pinched ends which extend beyond the sleeve. Each pinched end is fitted with a metal strap which is welded to the frame members. Current is supplied to the arc tube by a braided wire for the lower electrode and a flying lead well spaced from the arc tube for the upper electrode. The construction is especially suitable for a 1000 watt lamp.
The arrangement of the prior art requires two rings for mounting the sleeve and two straps for mounting the arc tube, which is four metal components requiring at least four welds for fixing to the frame members. Since the pinched ends of the arc tube extend beyond the protective sleeve, it is possible that shards of arc tube will impinge the outer envelope if the lamp fails.
U.S. Pat. No. 5,729,078 also discloses a discharge lamp wherein an arc tube and the surrounding shield are supported by a metal frame mounted to the stem. The frame is directly connected to leads in the stem. For high wattage lamps, which have larger components, this would place undue stress on the stem. Since the frame is not electrically isolated, sodium loss would be unacceptable in high wattage lamps. Further, the pinched ends of the arc tube still extend beyond the shield.
In order to reduce manufacturing costs in a lamp having an electrically floating frame, it would be preferable to reduce the number of parts and the number of welding steps. From a safety standpoint, it would also be preferable for the protective sleeve to extend beyond the ends of the arc tube.